Code Red only affects versions 4.0 and 5.0 of the IIS software. Windows 95, 98 or Me are not affected, nor are computers running Unix, Linux, or Mac operating systems.

Netcraft, which carries out regular surveys of web server software, estimates that around 3.5 million sites are using Microsoft IIS software. Of these, about 35% were initially vulnerable, a figure that has now dropped to 15% following the publicity about the worm.

Worms are becoming the weapon of choice among virus writers because worms spread on their own rather than requiring computer users to spread them.